Energy from Reactions
Photosynthesis is an endothermic reaction.

Energy from Reactions

Calculating the energy changes in chemical reactions can be a little worrying but the only maths skills required are addition and subtraction ... you can slip in a little multiplication if you wish!! The only chemical skill you need is to be able to work out what bonds are present in each substance but in the GCSE Chemistry exam, there will be plenty of clues in the question.

It pays to be organised, even in the exam. Take a sheet of paper and draw a vertical line down the centre. The left column will be for the reactants and the right for the products. Start by writing down the individual bonds in the reactants. Next, write down how many of each bond there is. Then write down the bond energy for each one and calculate the energy contained in that number of each bond. Then work out total energy contained in the bonds of the reactants by adding the individual totals together. Do this step by step and double check at each stage to eliminate any errors.

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Repeat the process for the products. Then subtract the total bond energy of the reactants from the products, paying careful attention to whether the answer is a positive or a negative number. Positive numbers mean that the reactions are endothermic, negative numbers mean that they are exothermic chemical reactions.

The questions that follow concern making and breaking bonds and the energy changes involved in doing so. To be able to complete these questions you will need the following bond energy information and a calculator:

  • C-H 413kJ/mol
  • O=O 498kJ/mol
  • C-O 358kJ/mol
  • O-H 464kJ/mol
  • N≡N 945kJ/mol
  • H-Cl 432kJ/mol
  • Cl-Cl 243kJ/mol
  • H-H 436kJ/mol
  • C=O 805kJ/mol
  • C-C 347J/mol
  • N-H 391kJ/mol.

Now try these ten questions on energy changes - good luck!

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  1. Pick the correct combination for the correct information regarding bond energies.
    Remember EXothermic EXit and ENdothermic ENter
  2. The overall energy change in a chemcial reaction is -24kJ/mol. What does this tell us about the reaction?
    A negative energy change shows the reaction is exothermic and so heat and/or other energy has been transferred into the surroundings, often increasing the temperature
  3. Using the bond energies provided, calculate the energy change for this reaction.
    2CH4 + O2 → 2CH3OH
    When carrying out these calculations, always subtract the reactants energy from the products
  4. In the Haber process, nitrogen and hydrogen are reacted together to produce ammonia. Use the bond energies provided to calculate the overall energy change in the reaction.

    N2 + 3H2 ⇌ 2NH3

    945 + (3x436) - (6x-391) = -93kJ
  5. Use the bond energies provided to calculate the energy change in the following reaction.

    H2 + Cl2 → 2HCl

    Bond breaking = 436+243kJ/mol; bond making 2x432kJ/mol
  6. Use the bond energies provided to calculate the energy change in the following reaction.

    oxygen + hydrogen → water

    Add together the energy of each bond of the reactants, do the same for the bonds in the products. Then subtract the total bond energy of the reactants from that of the products to get the answer
  7. Use the bond energies provided to calculate the amount of energy given out when propane is burnt.

    C3H8 + 5O2 → 3CO2 + 4H2O

    Remember that CO2 has 2 C=O double bonds not single bonds
  8. If ΔH for a reaction is negative, what does this tell us about the reaction?
    Remember to always subtract reactants from products - the total bond energy of the products must be less than the reactants to end up with a negative answer
  9. Use the bond energy values provided to calculate ΔH for the reaction of photosynthesis.
    6CO2 + 6H2O → C6H12O6 + 6O2

    Hint: For glucose (cyclical) there are the following bonds formed: 7xC-H, 5xC-C, 7xC-O and 4xO-H.
    Photosynthesis is an endothermic reaction and therefore ÄH is positive. LHS total = 15228kJ RHS total = 11976 LHS - RHS = ΔH
  10. What is the value for ΔH for respiration?
    Respiration is the same as photosynthesis, but in the opposite direction!!

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